Molecular weight degradation stabilized polymeric compositions

ABSTRACT

Polymeric compositions are stabilized against molecular weight degradation by the inclusion in the composition of an inhibitor which is preferably an ethylenically unsaturated compound. The invention is of particular value when the polymer is a polymer of (meth)acrylamide in which the amount of contamination of the polymer with free (meth)acrylamide monomer is extremely low and the inhibitor has LD 50  above 400.

This invention relates to water soluble and water swellable polymers ofethylenically unsaturated monomers and their stabilisation so as toreduce molecular weight degradation during storage and use. It alsorelates to polyacrylamides in which this problem of degradation isparticularly significant.

When making a water soluble or swellable polymer from ethylenicallyunsaturated monomer, it is normal to try to achieve full polymerisationof the monomers but in reality some monomer always remain unpolymerisedin the polymeric reaction product. This may be relatively unimportant inthe case of some monomers but it is known to be undesirable in the caseof methacrylamide or, especially, acrylamide because of the toxicity ofthis monomer. It is normal practice therefore to conduct acrylamidepolymerisation so as to reduce the acrylamide content to as low a valueas is conveniently possible, which in practice generally means that thepolymer has a residual free acrylamide content of, typically, 0.2 to0.5%.

Extensive studies have been made of the performance of polymers ofethylenically unsaturated monomers and it is known that they are liableto undergo degradation during storage or use. This degradation isparticularly serious for the higher molecular weight polymers, forinstance molecular weight above one million. The fact that degradationis occuring is manifested by, for instance, a reduction in the solutionviscosity of the polymer. In those circumstances when the polymer isbeing used as a viscosifier the reduction in viscosity would probably benoticed, but even then this might be put down to other factors, forinstance that the polymer as initially made had lower solution viscositythan was expected. Also, it may not be practical to measure solutionviscosity during use (e.g. when the polymer is being used downhole). Inother situations, e.g. as a flocculant, the solution viscosity of thepolymer may not normally be measured and the performance of the polymermay depend upon a whole range of factors of which solution viscosity isonly one, and so in theory performance could be put down to any of thesefactors and, again, may be attributed to inferior properties in thepolymer as manufactured initially.

Even when it is observed that solution viscosity has been reduced thisreduction could be due to changes in the side groups, for instance,hydrolysis, or could be due to cleavage of the backbone, i.e. reductionof molecular weight, and it can be relatively difficult to prove clearlywhich effect is occurring.

Despite these uncertainties as to the cause of degradation, there havebeen numerous proposals to incorporate various degradation inhibitors inpolyacrylamides. Examples are given in the following Chemical Abstracts,namely isobutanol, trichlorphenolate and amino acids in volume 108189657g, phosphonates in volume 106 511875f, N-methyl-2-pyrrolidone involume 105 229619t, maleic anhydride acylation derivatives of urea,thiourea, phenylurea or ethanolamine in volume 99 140769b, varioussulphur compounds such as thiocarbonates in volume 56 52294m, thioureaand polyethylene glycol in volume 91 23783g and various compounds suchas napthoquinone in volume 98 108362g. Many of the additives aredescribed as being added to prevent oxidation of the polymer and inChemical Abstract 88 23892y an inorganic reducing agent is used.

In FR-A-2604444 the viscosity of a polyacrylamide for enchanced oilrecovery is stabilised by adding at least 5% acrylamide monomer, base onthe polymer. In JP-A-60/210657 it is proposed to stabilisepolyacrylamide homopolymers, and copolymers of acrylamide with less than50% of the monomers, that are to be used for purpose such asflocculation, paper-making, enhanced oil recovery, viscosifiers and soilimprovers. This stabilisation is by the addition of at least 0.5% of awater soluble vinyl monomer and in the examples the monomers used areacrylamide (in an amount up to 7%) sodium acrylate, methacrylamide,acrylonitrile, dimethylaminoethyl acrylate and2-acrylamido-2-methylpropane sulphonic acid (AMPS, U.S. Trade Mark).

Despite all this extensive literature the commercial reality is that avery limited range of additives are incorporated to improve stability.Urea is included for various reasons an can give some improvement.Greater improvement is achieved with thiourea, sodium nitrite ortrimethoxyphenol but each of these materials are rather inconvenient toincorporate into the polymer, and the polymer is still liable to undergosubstantial viscosity reduction during storage and use, especially atelevated temperatures.

Separate from the stabilization of conventional polyacrylamide,contaminated with, for instance, 0.2% or more free acrylamide, somepolyacrylamides contaminated with less free acrylamide have beenproduced. For instance polymer that is to be used for the purificationof potable water generally has a free acrylamide content of around0.05%. The viscosity of such a polymer is not directly significant forwater-purification properties and Iam unaware of any viscosity or otherstability studies having being conducted on such polymers.

It is also known to make polyacrylamides having even lower contents offree acrylamide. This can be achieved either by careful optimisation ofthe polymerisation conditions or by removal of the free acrylamide afterpolymerisation by washing, chemical reaction or biological means, forinstance as described in European Patent Applications 89301185.8 and89301186.6 or U.S. Pat. No. 4,687,807.

Thus the present industrial situation is that all polyacrylamides havingtypical levels of acrylamide contamination are liable to undergoviscosity degradation on storage or use under some conditions, thecommercially used ways of trying to prevent this degradation are notfull satisfactory and incur various disadvantages, methods in theliterature also appear unsatisfactory, FR-A-2604444 and JP-A-60/210657describe the stabilisation of acrylamide polymers by the use ofacrylamide or other monomers but toxicity considerations contraindicatethe general teachings of these patents (for instance the use of largeamounts of acrylamide) and relatively pure polyacrylamides (having a lowcontent of free acrylamide) are even more unstable.

In a first aspect of the invention, I have now found that it is possibleto stabilise these relatively pure polyacrylamides against molecularweight degradation and that this can be achieved easily by theincorporation of materials that are easy to incorporate and that do notreintroduce the toxicity problems that had been avoided by the reductionof the content of free acrylamide.

In particular, the first aspect of the invention provides a polymericcomposition comprising of blend of a polymer of (meth) acrylamide, free(meth) acrylamide monomer and viscosity-degradation inhibitor, and inthe invention the amount of free (meth) acrylamide monomer is below 0.1%by weight of the polymer and the LD₅₀ of the inhibitor is above 400.

LD₅₀ of the above 400 means that more than 400 mg of the inhibitor isrequired per kilogram bodyweight to achieve 50% lethality whenadministrated orally to rats.

Thus, relative to conventional commercial polyacrylamide, the product ofthe invention omits most of the relatively toxic (meth) acrylamide(thereby making the product potentially very susceptible to viscositydegradation) and replaces this monomer by a material that is much lesstoxic and that does stabilise the polymer against viscosity degradation.

The preferred polymeric compositions have a stability against viscositydegradation at least as good as and preferably better than the samepolymer containing 0.2% free (meth) acrylamide.

Despite having this equivalent and, preferably, greatly improvedstability the overall toxicity (measured both as LD₅₀ and as cumulativetoxicity) of the stabilised composition can easily be no greater thanpolyacrylamide containing 0.2% free (meth) acrylamide and is preferablysubstantially less. Thus, it is possible, by the invention, for thefirst time, to provide a polyacrylamide that meets the highestenvironmental standards as regards free (meth) acrylamide and yet hasexceedingly good viscosity stability.

Despite all the discussion in the literature about oxidation inhibitorsand side group reactions, it appears that viscosity degradation isprimarily due to reduction in molecular chain length, and that this inturn is due to breakage of the polymer backbones by unreactedpolymerisation initiators, and degradation products of such initiators,left over from the polymerisation process. In particular the presence offree radical initiators in the final polymer seems to be a particularcause of molecular weight degradation and thus viscosity degradation.

The amount of degradation inhibitor that has to be added, to achieve anyparticular stability, is therefore reduced if the amount of initiatordebris in the polymer is reduced. Preferably therefore thepolymerisation is conducted using techniques that result in the lowestpossible initiator residues in the final product. For instance, the rateof addition of initiator is preferably controlled so as to minimiseinitiator residues in the final polymerisation product. If the polymeris substantially free of such residues then the amount ofviscosity-degradation inhibitor that has to be added can be relativelylow. It is then possible to use any of the degradation inhibitors thatare known from the literature and which both function as a free radicalsink and which have LD₅₀ above 400.

Preferably, however, the inhibitor is an ethylenically unsaturatedmaterial that has LD₅₀ above 400.

Acrylamide itself (LD₅₀ =124) is of course excluded from considerationas are other monomers that have similar or worse toxicity values, suchas acrylonitrile (LD₅₀ =78) and acrylic acid (LD₅₀ =250).

Monomers that can be used include low toxicity ethylenically unsaturatedcarboxylic acids, for instance maleic acid (LD₅₀ =708) and fumaric acid(LD₅₀ =10700) and ethylenically unsaturated cationic monomers,especially dialkyl aminoalkyl (meth) acrylates such as dimethylaminoethyl (meth) acrylate (LD₅₀ =1751) and dialkyl aminoalkyl (meth)acrylamides especially those where the central alkylene group containsat least two chain carbon atoms, for instance methyl chloride quatenarysalt of dimethyl aminopropyl (meth) acrylamide (Maptac, LD₅₀ =715).Diallyl dialkyl monomers can be used especially diallyl dimethylammonium chloride (Dadmac, LD₅₀ =1700).

A wide variety of other ethylenically unsaturated monomers are of coursewell known and potentially useable, for instance, monoallyl triallylquaternary compounds such as allyl trimethyl ammonium chloride (atmac),other allyl materials such as allyl sulphonate, other anionic materialssuch as vinyl sulphonate and styrene phosponate, maleic anhydride,fumaric anhydride, dimethyl amino ethyl acrylate and dimethyl aminopropyl acrylamide.

The use of monomers containing one or more allyl groups is particularlypreferred, especially Dadmac.

The ethylenically unsaturated material does not have to be a monomer andit can instead be a material that contains a polymeric backbone withethylenic unsaturation either in the backbone, in terminal groups or inpendant groups. The use of a polymer in this manner is particularlydesirable since it can contribute to the viscosity or other propertiesof the polymer that is being stabilised or, at least, minimise thedilution effect on the polymer that might be provided otherwise by themonomer. Preferred polymers are polymers formed from allylic monomer(either alone or with up to 90%, preferably not more than 50%, otherethylenically unsaturated monomer. Preferred allylic monomers arediallyl dialkyl ammonium compounds. The polymers of diallyl dimethylammonium chloride (especially the homo polymer) are particularlypreferred in this respect. Such polymers are believed to be terminatedby free allyl groups.

Other suitable polymers are acrylic terminated polyethylene glycols andother unsaturated polymers including, especially, the prepolymersdescribed in European application 89301075 (EP-A-328321).

It can be acceptable for the ethylenically unsaturated material toundergo polymerisation during the storage or use conditions that wouldotherwise have caused viscosity degradation, but it is generallypreferred that the material merely acts as a free radical sink and doesnot undergo any substantial chemical change as a result of this. Onedisadvantage of the monomer being too reactive is that it may tend toreact onto the polymer itself and this may alter the performancecharacteristics of the polymer. In some instances this is undesirable.For instance if the polymer is soluble, it may tend to be insolublisedby reaction of a monomer that is too reactive. Amps is a example of amonomer that tends to have too high a reactivity if it is to be used inan environment where some degree of insolubilisation is unacceptable.

The material may be non-ionic or it may be co-ionic or counter-ionicwith the polymer. If it is counter-ionic the amount must not be suchthat the monomer forms an insoluble complex with the polymer. Theinhibitor should normally be water soluble so that it can be introducedas an aqueous solution.

The polymer that is to be stabilised in this first aspect of theinvention is formed from (meth) acrylamide alone or with otherethylenically unsaturated monomer. When there is a blend of monomers,the blend (and usually each of the monomers in the blend) preferably iswater soluble.

The polymer that is to be stabilised is preferably soluble in aqueousliquid, but the invention can also be applied to polymers that areinsoluble, for instance as a result of being cross-linked bypolyethylenically unsaturated monomer or other suitable crosslinkingagent. If it is insoluble, it may be crosslinked to such an extent thatit is substantially non-swelling but generally it is relatively lightlycrosslinked such that it is swellable and has high absorption capacity,for instance at least 30 grammes de-ionised water per gramme polymer.

The polymer may be a substantial homopolymer of (meth)acrylamide,generally polyacrylamide homopolymer in which event it should benon-ionic. However, it may be anionic as a result of a small degree ofhydrolysis of acrylamide groups or as a result of copolymerisation ofthe (meth)acrylamide with ethylenically unsaturated carboxylic orsulphonic monomer. Suitable monomers are any of the conventional watersoluble anionic monomers such as (meth)acrylic acid (generally as awater soluble salt) or AMPS.

The polymer may be cationic, in which event it is co-polymerised withethylenically unsaturated cationic monomer. Suitable cationic monomersinclude dialkylaminoalkyl(meth)-acrylates, -methacrylates, -acrylamides,and -methacrylamides (especially when the central alkylene groupcontains at least 2 chain atoms) and diallyl dialkyl monomers. Preferredexamples are any of the cationic monomers mentioned above, in particulardimethyl(or diethyl)aminoethyl(meth)acrylates and dimethyl(ordiethyl)aminopropyl(meth)acrylamides, and Dadmac. The (meth)acrylatesand (meth)acrylamides will normally be present as acid addition orquaternary salts. The amount of (meth)acrylamide in the monomers fromwhich the polymer is formed may range from 5 to 100% by weight, often 50to 100%.

The polymer generally has molecular weight above 1 million, often above5 million. It generally has intrinsic viscosity (measured in 1 molarsodium chloride at 25° C. by suspended level Viscometer) above 4 dl/g.When the polymer is anionic the IV is typically in the range 10 to 30.When the polymer is cationic the IV is typically in the range 6 to 18.

The polymer may be made by polymerisation in conventional mannerconducted with careful optimisation so as to produce a residual freeacrylamide content of below 0.1% preferably below 0.05% and mostpreferably below 0.03%. Methods currently used for making polymer forpotable water treatment may be utilised. Alternatively the polymer maybe synthesised and may then be subjected to washing or otherpurification, as discussed above, so as to reduce the monomer content.

The invention is of particular value when applied to cationic polymersbecause it is well known that they tend to suffer significant reductionin viscosity during many of their conventional uses, for instancedownhole acidising, fracturing or completion fluids, and the inventionprovides a significant commerically and toxicologically effectivesolution to this problem.

The inhibitor can be present throughout the formation of the polymer ifit has a sufficiently low reactivity rate that it will not participatein or interfere with the polymerisation reaction. Generally, however, itis added after the polymerisation is completed. If the polymer ispresent as a fluid solution or emulsion or dispersion (for instance areverse phase dispersion or emulsion) the inhibitor can conveniently beincorporated into this fluid composition merely by stirring.

A particularily preferred method of blending the inhibitor with a watersoluble or water swellable polymer comprises providing the polymer as adispersion of particles in an non-aqueous liquid (for instance bydispersing chopped gel into oil or, preferably, reverse phasepolymerisation) and then mixing the inhibitor into the dispersion, theinhibitor and the non-aqueous liquid being selected such that theinhibitor is preferently soluble in the particles. Preferably theparticles in the dispersion are substantially dry before the inhibitoris added. For instance a dispersion may be made by reverse phasepolymerisation followed by azeotroping, and inhibitor (generallydissolved in water) may then be mixed into the dispersion.

If the polymer is provided as a solid gel, for instance as beads orcomminuted gel particles, the inhibitor, (generally as an aqueoussolution) may be imbibed into the particles either before they are driedor after drying (in which event a further drying step maybe necessary).

The inhibitor may serve to prevent or reduce viscosity undersubstantially any conditions during which such degradation is likely tooccur, e.g. upon heating, exposure to ultra violet radiation ordaylight, dilution waters or other liquors that contain impurities (e.g.ferrous iron) that cause viscosity degradation, or prolonged storage.The inhibitor may serve to prevent or reduce viscosity degradation inany type of composition e.g. a dry product such as bead or comminutedgel, a reverse phase dispersion, or a solution, for instance flocculantsolution between initial makeup and use as a flocculant.

The compositions of the invention can be used for a wide variety ofpurposes, depending on the particular polymer. For instance such usescan be selected from viscofication, enhanced oil recovery, flocculation,paper making, soil improvers, wallpaper and other adhesives, watershut-off and soil stabilisation and grouting, absorbents and soforth.

It is of particular value when the polymer is to be subjected toheating, hot and/or chemically aggressive conditions, e.g. as flocculantfor Bayer process liquors.

The polymer can be used in various downhole environments. For instancethe polymer may be a high molecular weight anionic polymer for enhancedoil recovery. However, the inventions is of particular value in hotaggressive downhole environments, as described in PCT applicationGB/90,100,773 filed today by the same applicant and inventor. Thus itmay be a high molecular weight anionic polymer (generallycross-linkable) for downhole fracturing uses, a high molecular weightanionic or cationic polymer for downhole acidising, a medium to highmolecular weight cationic polymer for downhole completion fluids, or amedium molecular weight anionic polymer as a fluid loss additive indrilling fluids.

The polymer may be one that is to be reacted, at its pendent groups, atelevated temperature after its formation and the inhibitor minimisesdegradation during reaction. For instance polyacrylamide can behydrolysed by heating in aqueous alkali and degradation during thehydrolysis can be minimised in the invention.

The polymer may be exposed for prolonged periods to an electrolyte, forinstance as in a textile printing paste. The polymer may be exposed forlong periods to ultraviolet radiation, for instance as an agriculturalpolymer for enchancement of soil structure. The polymer may be exposedto high temperatures for a short period, for instance it may be a wallpaper prepaste adhesive and molecular weight degradation during hotembossing or foaming stages of the wallpaper can be minimised by thepresence of the inhibitor of the invention.

The amount of inhibitor that has to be added will be found by trial andexperiment. It usually at least 0.05% and generally at least 0.1%. Sincethe inhibitor may tend to dilute the activity of the polymer it isgenerally preferred that the amount should be below 20%, preferablybelow 10% and most preferably below 5% by weight of the polymer.

However, when a dilution effect does not create a problem or is evendesirable (for instance when the inhibitor is a polymer, or duringstorage forms of polymer, that complements the properties of the polymerthat is to be stabilised) then larger amounts may be used, e.g. up to50% by weight.

Some of the discoveries on which the first aspect of the invention arebased are applicable also to other compositions, for instance tocompositions where the monomers from which the polymer is made are freeof (meth)acrylamide and/or to polyacrylamides that are contaminated withup to, for instance, 0.5% or even 1% by weight free (meth)acrylamidemonomer. The invention therefore includes also other aspects which areapplicable to such polymers, as well as to the polymers defined in thefirst aspect of the invention.

A second aspect of the invention resides in effective ways ofincorporating the inhibitor with the polymer that is to be stabilised.In JP-A-60/210657 and FR-A-2604444 the inhibitor is added to a solutionof the polymer, but this renders the inhibition technique inapplicableto solving the problem of storage stability of dry polymers orsuspension polymers, and these are the forms in which such polymers aremost usually supplied commercially. I have surprisingly found that theinhibitor can be added to the polymer while in the form of a gel, apowder or a dispersion in water immiscible liquid. Suitable methods aredescribed above.

The only proposals in the literature for the use of monomers of the typediscussed above (JP 60210657 and FR 2604444) relate to the stabilisationof acrylamide polymers. In a third aspect of the invention, I havesurprisingly found that it is possible to stabilise, in a similarmanner, polymers made from monomers that are entirely free of(meth)acrylamide. Thus, it is now possible to stabilise polymers inwhich 100% of the monomers are anionic or cationic. Suitable anionic andcationic monomers are those listed above for incorporation in theacrylamide polymers of the first aspect of invention. Copolymers of themwith other non-ionic monomers, for instance vinyl acetamide,N-vinyl-N-methyl acetamide, N,N-dimethylacrylamide or vinyl pyrrolidonecan be used.

Preferably the polymers are formed from monomers selected from AMPS or,preferably, dialkylaminoalkyl methacrylates or dialkylaminoalkyl(meth)acrylamides where the central alkylene group contains at least 2carbon atoms, as discussed above.

As indicated above, preferred results are achieved when the stabilisingmonomer is allylic, either an allylic monomer or a polymer made frommonomer comprising allylic monomer, and according to a fourth aspect ofthe invention any of the polymers discussed above are stabilised by theuse, as inhibitor, of an allylic monomer or polymer, preferably Dadmac.

In a fifth aspect of the invention, any of the polymers discussed aboveare stabilised by the addition of a polymer that includes ethylenicunsaturation, for instance, the allylic polymers or other ethylenicallyunsaturated polymers mentioned above.

In the second and third aspects of the inventions, the inhibitingethylenically unsaturated monomer is preferably one of those discussedabove for use in the first aspect of the invention but can,alternatively, be other monomers such as acrylamide itself,acrylonitrile or acrylic acid. The products of these second to fifthaspects of the invention may otherwise be formulated using the samematerials, and for the same purposes as are discussed above for thefirst aspect of the invention.

EXAMPLE 1

The viscosity (Brookfield RTV, spindle 1 to 20 rpm) of a polymersolution aged at 150° C. for various times and the same solution towhich 1,000 ppm DADMAC had been added was measured. The aging time andviscosity results are shown below.

    ______________________________________                                               Blank         Dadmac                                                          Viscosity                                                                             %         Viscosity %                                                 (CP)    Retained  (CP)      Retained                                   ______________________________________                                        Initial  113       --        86      --                                       1 hour @ 116       102.7%    104     120.9%                                   150° C.                                                                2 hours @                                                                              86        76.1%     98.5    114.5%                                   150° C.                                                                4 hours @                                                                              66        58.4%     84      97.7%                                    150° C.                                                                16 hours @                                                                             47.5      42.0%     86.5    100.6%                                   150° C.                                                                ______________________________________                                    

EXAMPLE 2

When the process of example 1 was repeated using the co-polymer of 60parts sodium Amps, 32.5 parts by weight acrylamide and 7.5 parts byweight sodium acrylate, the addition of 1000 ppm dadmac again gave adramatic improvement in viscosity retention.

EXAMPLE 3

To demonstrate the potential benefit of various inhibitors on polymersmade from ethylenically unsaturated monomers and which are notcontaminated with any acrylamide at all, a laboratory test was developedobserving the viscosity changes from initially making up a 1% solutionof the polymer in 15% aqueous hydrochloric acid followed by storing itfor one hour at 200° C. The polymer for this laboratory test was ahomopolymer of dimethyl aminoethyl (meth) acrylate (dmaema) quaternisedwith methyl chloride. The process was conducted for each inhibitor at ainhibitor dosage of 100 ppm and at a inhibitor dosage of 500 ppm. Thepolymer to which no inhibitor had been added underwent 63.5% viscositydegradation during the test. Thus any value higher than this is animprovement. The values in the presence of the various stabilisers areshown in table 1 below. Viscosity was measured as in Example 2.

    ______________________________________                                                        % Viscosity                                                                   Retained                                                      Degradation Inhibitor                                                                           100 ppm  500 ppm                                            ______________________________________                                        Blank             63.5%    63.5%                                              Dadmac            69.3%    83.3%                                              Atmac             75.6%    79.8%                                              Allyl Sulphonate  65.5%    71.9%                                              Vinyl Sulphonate           78.6%                                              Maleic Anhydride  76.3%    71.8%                                              Amps (Sodium Salt)                                                                              43.2%    38.6%                                              Dmaema            73.6%    78.6%                                              Aptac             74.9%    91.4%                                              Acrylic Acid      79.2%    75.2%                                              Acrylamide        79.8%    86.0%                                              Acrylic Pre-Polymer*                                                                            67.0%    65.9%                                              ______________________________________                                         *This is a prepolymer according to the Example of European Application        89301075.                                                                

I claim:
 1. A polymeric composition comprising a blend formed by mixinga preformed polymer of (meth)acrylamide and containing residual free(meth)acrylamide monomer with viscosity-degradation inhibitor,characterized in that the amount of free (meth)acrylamide monomer isbelow 0.1% by weight of the polymer and the LD₅₀ of the inhibitor isabove 400 mg/kg in rats and the oral inhibitor is an ethylenicallyunsaturated monomer which in the composition does not undergo anysubstantial reaction with said polymer.
 2. A composition according toclaim 1 in which the inhibitor is selected from ethylenicallyunsaturated cationic monomers, allylic monomers, maleic acid oranhydride, fumaric acid or anhydride, vinyl sulphonate, and styrenephosphonate.
 3. A composition according to claim 1 in which theinhibitor is an allylic monomer.
 4. A composition according to claim 1in which the inhibitor is diallyl dimethylammonium chloride.
 5. Acomposition according to claim 1 in which the polymer of acrylamide hasintrinsic viscosity above 4 dl/g and is selected from acrylamidehomopolymer and copolymers of acrylamide with anionic or cationicethylenically unsaturated groups.
 6. A composition according to claim 1in which the acrylamide polymer has intrinsic viscosity above 4 dl/g andis a copolymer with a monomer selected fromdialkylaminoalkylmethacrylate, dialkylaminoalkyl(meth)acrylamide wherethe central alkylene groups contains at least 2 carbon atoms, and2-acrylamido-2-methylpropanesulphonate.
 7. A composition according toclaim 1 in which the acrylamide polymer is a copolymer of acrylamidewith dimethyl or diethyl aminoethylmethacrylate or dimethyl diethylaminopropyl(meth)acrylamide.
 8. A method of making a composition thatcomprises a blend of a viscosity-degradation inhibitor and a preformedpolymer formed by polymerization of a water soluble ethylenicallyunsaturated monomer or monomer blend, characterized in that theinhibitor is an ethylenically unsaturated monomer which in thecomposition does not undergo any substantial reaction with said polymerand is added to the polymer either by imbibing an aqueous solution ofthe inhibitor into solid gel particles of the polymer before or afterdrying, or by mixing an aqueous solution of the inhibitor into adispersion of the polymer in particulate form in a non-aqueous liquid.9. A polymeric composition comprising a blend formed by mixingviscosity-degradation inhibitor with a preformed polymer formed fromwater soluble ethylenically unsaturated monomer or monomer blendcharacterized in that the inhibitor is an allylic monomer which in thecomposition does not undergo any substantial reaction with said polymer.10. A polymeric composition comprising a blend formed by mixingviscosity-degradation inhibitor with a preformed polymer formed fromwater soluble ethylenically unsaturated monomer or monomer blendcharacterized in that the inhibitor is an ethylenically unsaturatedmonomer which in the composition does not undergo any substantialreaction with said polymer and the polymer is formed from monomers thatconsist essentially only of ethylenically unsaturated cationic oranionic monomers.
 11. A method according to claim 8 in which theinhibitor is diallyl dimethylammonium chloride.
 12. A method accordingto claim 8 in which the polymer is formed of monomers that comprisedialkylaminoalkylmethacrylates and dialkylaminoalkyl(meth)acrylamideswherein the central alkylene group contains at least 2 carbon atoms. 13.A composition according to claim 9 in which the inhibitor is diallyldimethylammonium chloride.
 14. A composition according to claim 9 inwhich the polymer is formed of monomers that comprisedialkylaminoalkylmethacrylates and dialkylaminoalkyl(meth)acrylamideswherein the central alkylene group contains at least 2 carbon atoms. 15.A composition according to claim 10 which the inhibitor is diallyldimethylammonium chloride.
 16. A composition according to claim 10 inwhich the polymer is formed of monomers that comprisedialkylaminoalkylmethacrylates and dialkylaminoalkyl(meth)acrylamideswherein the central alkylene group contains at least 2 carbon atoms. 17.A polymer composition according to claim 1 in which the amount ofinhibitor is at least 0.05% by weight of the polymer.
 18. A polymercomposition according to claim 9 in which the amount of inhibitor is atleast 0.05% by weight of the polymer.
 19. A polymer compositionaccording to claim 10 in which the amount of inhibitor is at least 0.05%by weight of the polymer.
 20. A method according to claim 8 in which theamount of inhibitor is at least 0.05% by weight of the polymer.
 21. Amethod according to claim 8 in which the inhibitor has an oral LD₅₀above 400 mg/kg in rats.
 22. A method according to claim 8 in which theinhibitor is an allylic monomer.
 23. A polymeric composition accordingto claim 20 in which the inhibitor has an oral LD₅₀ above 400 mg/kg inrats.
 24. A polymeric composition according to claim 10 in which theinhibitor is an allylic monomer.